Gallium arsenide (GaAs) semiconductor devices have important applications for generating power in the high RF and microwave regions of the electromagnetic spectrum. GaAs heterojunction bipolar transistors (I-IBTs), in particular, have characteristics that make them especially suitable for power generation.
The power rating of an HBT (i.e., its current carrying capability) is directly proportional to the active surface area of the device. Because an HBT is a vertical device, its active area is determined by the surface area of the emitter. Depending on its design, a typical HBT unit cell has an active area of approximately 20 to 60 .mu.m.sup.2. An HBT of this size is able to handle about 1 milliwatt of RF power per square .mu.m of active area. Therefore, about 0.06 watts is typically the most power that can be generated by a single HBT cell. Attempts to increase the power much beyond this limit usually produce catastrophic failures.
To be useful as power devices, HBTs must be able to generate power on the order of watts rather than milliwatts. Such high power output levels are generated by increasing the effective area of the device. However, it is impractical to increase the active area of an HBT unit cell because current crowding (hot spots of high current) and detrimental heating effects negate any gain obtained by the increased area. The generally accepted alternative, which works well for small and medium devices generating less than about 0.25 watts, is to connect many HBT unit cells in parallel to increase the effective area.
With larger devices, a different form of current crowding, termed "thermal runaway," begins destroying the HBT cells. Thermal runaway occurs because the parallel devices are not all identical and do not have identical heat sinks. When one HBT cell gets slightly hotter than its neighboring cells, it begins to carry more current, which makes it hotter, so it carries more current, which makes it hotter, etc. This unstable condition rapidly leads to destruction of the entire device. As each unit cell is blown open, the remaining cells have to carry more current, which makes them hotter and more likely to be destroyed. A generally accepted method of controlling the thermal runaway problem is using thin film ballasting resistors with each cell to force uniform current distribution. It is also possible to grow an additional epitaxial layer on top of the HBT emitter layer to act as a ballasting element. Additional fabrication steps are required with all of these prior methods, however, and it is difficult to meet material requirements. Thus, there is a need for a new type of HBT ballasting that does not require additional epitaxial layers or fabrication processing steps.